Pool fingers are rather rare stalactite-shaped formations that form subaqueously, in cave pools. They are known from several sites in Lechuguilla Cave, New Mexico but have been seen in caves all over the world.. I shot the photo below while diving in a large lake there, and was about 10 feet below the surface when I encountered them. These are thicker than most occurences as shown below it. While the exact origin of these forms is unknown, they are suspected to be calcified organic material, such as bacterial stringers. U-loops, such as seen in the bottom photo, are a common aspect of pool fingers.